1. Field of the Invention
This invention relates to an imaging apparatus and method, especially to a millimeter wave imaging system suitable for screening applications, such as security screening, and in particular to a personnel imaging system able to image dielectric materials.
2. Discussion of Prior Art
There is a growing interest in providing effective screening equipment for security applications or contraband detection. For instance archway metal detectors are well known for access control at airports or other secure areas to detect metallic objects concealed about a person. Such detectors are good at identifying the presence of a metallic object but offer no discrimination as to what that metallic object might be and are obviously limited to the detection of metallic objects. Thus they provide no security against non metallic items such as ceramic weapons or explosives. Nor do they allow any screening for non-metallic contraband.
There has therefore been a lot of interest in developing an imaging apparatus which allows images to be collected that can used in the detection and identification of concealed items. Imaging equipment operating at millimeter wavelengths has been proposed as most clothing is largely transmissive at such wavelengths allowing more substantial items underneath to be observed.
Passive millimeter wave systems have been proposed in which millimeter wave cameras simply collect radiation from a field of view to generate an image. Metallic objects have different emissivity and reflectivity characteristic to the human body and so the intensity of radiation received from a metallic object varies to that received from the human body.
To provide greater contrast, especially in indoor situations, active systems are also known wherein the field of view is illuminated with radiation at millimeter wavelengths.
Millimeter wave imaging systems generally provide good contrast between the human body and metallic objects but are generally less good at identifying non-metallic materials, such as plastics or glass.
U.S. Pat. No. 5,073,782 discloses an active millimeter wave imaging system. Here a plurality of oscillators illuminate a scene with incoherent radiation so as to avoid speckle. Preferably the illuminating radiation is of a certain defined polarisation. In one embodiment the illuminating radiation is linearly polarised in one plane and the detector array only receives radiation linearly polarised in an orthogonal plane. Radiation directly reflected from the scene, say by metallic objects, preserves its polarisation and so is not received by the detector array. Radiation from ceramic or plastic materials undergoes multiple reflections and thus the polarisation is randomised. Thus some radiation from these objects will be received. U.S. Pat. No. 5,227,800 discloses a similar millimeter wave imaging system.
International patent application WO 03/083508 discloses an imaging system which provides improved contrast by illuminating the scene at two different illumination levels. The radiometric temperature of an object depends on the radiation it emits, transmits and reflects. Two different objects, having different characteristics may nevertheless have the same total radiometric temperature under certain conditions and will therefore be indistinguishable. WO03/083508 discloses a device which images the scene at least two different illumination levels so as to generate more information about a scene.
It is therefore an object of the present invention to provide an imaging apparatus which mitigates at least some of the above mentioned disadvantages.